chore: add style guide [skip ci]

STYLE_GUIDE.md

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+# Code Style Guide
+
+Beyond using Cargo format and Clippy, there are some specific code style guidelines laid out in this document for different parts of the project.
+
+## `core/database` crate
+
+w.r.t. `model.rs` files
+
+- All struct definitions must be commented.
+  ```rust
+  /// Server
+  pub struct Server {
+    /// Name of the server
+    pub name: String,
+  ```
+- Struct definitions should not include derives unless necessary (if additional traits such as Hash are required) and instead use `auto_derived!` and `auto_derived_partial!`.
+  ```rust
+  auto_derived_partial!(
+    /// Server
+    pub struct Server { .. },
+    "PartialServer"
+  );
+  ```
+- `auto_derived!` macro accepts multiple entries and should be used as such:
+  ```rust
+  auto_derived!(
+    /// Optional fields on server object
+    pub enum FieldsServer { .. }
+
+    /// Optional fields on server object
+    pub enum FieldsRole { .. }
+  );
+  ```
+- If special serialisation conditions are required, such as checking if a boolean is false, use the existing definitions for these functions from the crate root:
+  ```rust
+  #[serde(skip_serializing_if = "crate::if_false", default)]
+  ```
+- `impl` blocks may be defined below the struct definitions and should be ordered in the same order of definition. Methods in the block must follow the same guidelines as traits where-in: methods are ordered in terms of CRUD, there are empty line breaks, and methods are commented.
+
+w.r.t. `ops` module for models
+
+- All traits must use a the name format `AbstractPlural` where Plural is the plural form of the collection. e.g. Servers
+- Traits defined must follow these guidelines:
+
+  - Methods are ordered in terms of CRUD, create-read-update-delete ordering.
+
+    ```rust
+    #[async_trait]
+    pub trait AbstractServerMembers: Sync + Send {
+      /// Insert a new server member into the database
+      async fn insert_member(&self, member: &Member) -> Result<()>;
+
+      /// Fetch a server member by their id
+      async fn fetch_member(&self, server_id: &str, user_id: &str) -> Result<Member>;
+
+      /// Update information for a server member
+      async fn update_member(&self, .. ) -> Result<()>;
+
+      /// Delete a server member by their id
+      async fn delete_member(&self, id: &MemberCompositeKey) -> Result<()>;
+    }
+    ```
+
+  - There should be an empty line break between each method declaration.
+  - All methods must have an appropriate comment.
+
+- When implementing the trait defined in `ops.rs` with each driver, the method declaration style should be the same for ease of searching: same ordering, same comments, same line breaks.